Detection and identification of pressure-sensor faults in electro-hydraulic (EHB) braking systems

ABSTRACT

A method of detection and identification of pressure sensor faults in an electro-hydraulic braking system of the type comprising a brake pedal, respective braking devices connected to the vehicle wheels and which communicate with electronically controlled proportional control valves in order to apply hydraulic fluid under pressure to the braking devices, respective pressure sensors for measuring the hydraulic pressures at the individual braking devices, a hydraulic pump driven by an electric motor, a high pressure hydraulic pressure accumulator fed by the pump for the provision of hydraulic fluid under pressure which can be passed to the braking devices via the proportional control valves in order to apply hydraulic fluid under pressure to the braking devices in proportion to the driver&#39;s braking demand as sensed at the brake pedal, and a supply pressure sensor for monitoring the hydraulic pressure supplied to the electronically controlled proportional control valves. In accordance with the method, three or more of the pressure sensors are arranged to be subjected to the same pressure and their readings measured and compared whereby to identify a sensor whose reading does not correspond to that of the others.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to co-pending United Kingdom PatentApplication No. 982589.6, and is a continuation of U.S. patentapplication Ser. No. 09/448,115, filed Nov. 24, 1999 and issuing as U.S.Pat. No. 6,360,592 on Mar. 26, 2002, the disclosure of which is hereinincorporated by reference.

BACKGROUND OF THE INVENTION

The present invention is concerned with the detection and identificationof pressure sensor faults within the context of electro-hydraulic (EHB)braking systems.

A typical EHB system for a vehicle comprises a brake pedal, respectivebraking devices connected to the vehicle wheels and which are capable ofbeing brought into communication with electronically controlledproportional control valves in order to apply hydraulic fluid underpressure to the braking devices, a hydraulic pump driven by an electricmotor, and a high pressure hydraulic pressure accumulator fed by thepump for the provision of hydraulic fluid under pressure which can bepassed to the braking devices via the proportional control valves inorder to apply hydraulic fluid under pressure to the braking devices inso called “brake by wire” mode in proportion to the driver's brakingdemand as sensed at the brake pedal.

The EHB system is controlled by an electronic controller (ECU) which,inter alia, controls the hydraulic pump to keep the pressure in thehydraulic pressure accumulator within specified limits.

The hydraulic pressure supplied to the electronically controlled valves(the “supply pressure”) is monitored by a supply pressure sensor. Thehydraulic pressure at the various braking devices is measured byindividual pressure sensors at these braking devices. An isolating valveis included whereby the hydraulic pressure accumulator can beselectively isolated from the pump and the remainder of the system.

Pressure sensors are key components of such EHB systems, being used inthe process of controlling pressure as well as in the detection ofsystem faults. Pressure sensors of the type in question converthydraulic pressure into electrical signals which are supplied to thesystem's ECU for control purposes. If one of these sensors develops afault, such that incorrect signals are supplied to the ECU, then controlerrors could result. Incorrect pressure readings therefore have thepotential to cause uncomfortable and/or inconsistent control of thenormal braking function, to impair the efficiency of control systemssuch as Autonomous Cruise Control (ACC) or Anti-Lock Brake Systems(ABS), to trigger false warnings of system failure, or to overlook theoccurrence of genuine system faults. For these reasons one wishes todetect even relatively small sensor errors so that effects such as theabove can be mitigated by effective countermeasures.

Some types of fault can be detected by the sensor's individual on-boardelectronics, but others are most easily (and economically) found bycomparison with the signals of another sensor subjected to the samehydraulic pressure. Sensor duplication is, however, uneconomic.

It is therefore one object of the present invention to enable faults tobe detected and identified by sensor signal comparison but withoutrequiring sensor duplication.

In accordance with the present invention, three or more pressure sensorsare arranged to be subjected to the same pressure and their readingsmeasured and compared whereby to identify a sensor whose reading doesnot correspond to that of the others.

Preferably, in the case of a four or more wheeled vehicle, all pressuresensors associated with the braked wheels, and the supply pressuresensor, are connected together, with all of the proportional controlvalves opened fully or substantially fully.

In one test arrangement, with the vehicle stationary and the accumulatorisolating valve open, a comparison of the readings from all sensors ismade under pressure.

In an alternative test arrangement, the comparison of the sensorreadings is made with the vehicle stationary and the accumulatorisolating valve closed.

The invention is described further hereinafter, by way of example only,with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a schematic illustration of one embodiment of anelectro-hydraulic braking system to which the present invention isapplicable;

FIG. 2 is a sequence flow diagram illustrating one embodiment of asystem for detection and identification of sensor faults; and

FIG. 3 is a sequence flow diagram illustrating one technique fordetecting but not identifying the presence of a faulty sensor.

DETAILED DESCRIPTION OF THE INVENTION

Referring first to FIG. 1, the illustrated EHB system comprises a brakepedal 10 with an associated sensor 12 for the acquisition of thedriver's braking demand. The driver's demand is transferred to anelectronic control unit (ECU), evaluated there, and used as the sourcefor the generation of electrical control signals for proportionalsolenoid control valves 14 a, 14 b, 14 c, 14 d, a hydraulic pump 16,wheel brakes 18 a, 18 b of one axle supplied with hydraulic fluid byelectrically actuated brake channels 20 a, 20 b and wheel brakes 18 c,18 d of the other axle supplied by electrically actuated channels 20 c,20 d.

Under normal braking conditions, brake pressure modulation in theelectrically actuated brake channels 20 a, 20 b, 20 c, 20 d is effectedin a known manner by means of the proportional solenoid control valves14 a, 14 b, 14 c and 14 d, the brake pressure being provided by apressure accumulator/reservoir 22 whose pressure is maintained by thepump 16 operated by an electric motor 18. Pressure sensors 24 a and 24 bmonitor the hydraulic pressure at the wheel brakes 18 a, 18 b of thefront axle and pressure sensors 24 c and 24 d monitor the hydraulicpressure at the wheel brakes 18 c, 18 d of the rear axle. Furtherpressure sensors 26, 28 monitor the pressure within push-throughcircuits 27 a, 27 b for the right and left front wheel brakes and apressure sensor 30 monitors the supply pressure in the circuit of pump16.

The operation of the aforegoing system is briefly as follows.

The accumulator 22 is maintained by the pump 16 within a specifiedpressure range, as observed by the supply-pressure sensor 30. Brakingdemands by the driver result in actuation of the proportional solenoids14 associated with each wheel. Energization current will be increased ifthe pressure observed by the associated sensor 24 is less than thatdemanded, and reduced if it is greater am the demanded pressure. Thenormal requirement will be for identical pressures to be present at theleft and right wheels of each axle, but the pressures at the front andrear axles may well be different in order to match the desired brakingdistribution. For this type of braking the master-cylinder isolationvalves 31 a, 31 b will be closed, the accumulator's isolation valve 32will be open, and the cross-axle-balance valves 34 a, 34 b can each beopened to ensure pressure equality left-to-right. However, someoperating modes require individual control of the pressure at eachwheel, e.g. during ABS or transverse pressure apportioning, and thecross-axle-balance valves 34 a, 34 b will be closed at such times.

This hydraulic layout has been devised in order to permit most sensorsto be compared with at least one other sensor under most operatingconditions. For example, the pressure signaled at the front left wheelcan be compared with that signaled at the front right wheel whenever thefront-axle balance valve is open, and the sensors for themaster-cylinder's primary and secondary circuits are normally subjectedto the similar pressures.

However, comparison of at least three sensor signals is necessary inorder to diagnose which signal is incorrect. Mere knowledge that a faultexists is insufficient for situations which require a fault-management(fall-back) strategy to be adopted. Also there is no convenient partnerfor the supply-pressure sensor 30.

Thus, for example, whenever the balance valves 34 a, 34 b are open it ispossible to detect errors by comparing the sensor readings from oppositeends of each axle, e.g. front left with front right. However this doesnot identify which sensor is in error, so any fault managementstrategies triggered by this test must take this uncertainty intoaccount. If more effective strategies are required then the diagnosismust be more specific.

A similar situation exists for the accumulator pressure sensor 30.During pump operation, one can compare the sensor signal with a signalrepresenting motor torque (proportional to pump delivery pressure) whichcan be obtained from observations of the motor behavior, but this willnot identify whether the fault is in the sensor or the motor.

Likewise during periods without braking, all four brake pressure sensors24 a, 24 b, 24 c, 24 d will be connected to the reservoir 39 and it ispossible to compare their signals at zero pressure, but this will notcapture faults which are only apparent under pressure. Also, thepressure at the supply-pressure sensor 30 is uncertain at such times. Ifthe accumulator isolating valve 32 is open then the supply-pressuresensor 30 will normally be at accumulator pressure. If the valve isclosed then it will be at some intermediate pressure due to inevitablepressure decay via internal leakage at the proportional control valves14.

The hydraulic layout of the system of FIG. 1 provides a solution tothese problems in that all four brake pressure sensors 24 a, 24 b, 24 c,24 d are connected together and to the supply pressure sensor 30, if allof the proportional valves 14 a, 14 b, 14 c, 14 d are opened fully.

This arrangement provides for the following test possibilities.

1. Open all proportional valves (14 a to 14 d) with accumulator valve(32) open.

This allows detection of zero drift and gain errors, including thosewhich might be caused by a sharp change (knee) part way along thesensor's characteristic of pressure to voltage. However, the highpressures reached (accumulator pressure) mean that there are issues ofsafety and reliability. This, for example, with the vehicle stationaryand accumulator isolating valve 32 open, a five-sensor (the four brakepressure sensors 24 and the supply pressure sensor 30) comparison ispossible under pressure. The high pressure involved will means however,that frequent checks in this manner could lead to premature caliperfailure due to fatigue. The test is thus useful for diagnosis, once afault has been detected by other means (e.g. the routine cross-axlecheck as described hereinafter with reference to FIG. 3) and could formpart of a system-initialization check but is not suitable for frequentlyinvoked fault detection. It must be made sure that this test is onlyused when the vehicle is stationary. The main use of this test would be:

a. to diagnose brake-pressure sensor faults already detected, so thatthe correct fault-management procedure can be implemented, and/or

b. to detect and diagnose accumulator pressure sensor faults.

2. Open all proportional valves (14 a to 14 d) with accumulator valve(32) closed.

This improves the safety and reliability concerns associated with 1.above, and can be used routinely every time the vehicle comes to rest.It can therefore diagnose faults already detected (by the simplecross-axle comparison test) at the earliest possible moment afterdetection. Such a test can be used frequently and has the additionaladvantage that it can use a range of pressures depending upon the brakepressure used for that stop, so that sensor abnormalities in the normalworking range are more easily noted.

However, because end-of-stop pressure is not usually very high(rarely>30 bar) this test is not effective for faults which occur onlyat higher pressures, e.g. a knee in the pressure/voltage characteristic.

3. Cross-axle and inter-axle sensor (24 a to 24 d) plausibility checkwith seal-friction offset.

This is a method of diagnosing which brake pressure is faulty bycomparing the pressures across the axle and between the front and rearaxles. This may appear to be a straightforward exercise to perform.However systems with isolating pistons (50 a, 50 b) for the push-throughcircuit have special problems because of the seal friction.

a. Comparing pressure signals across the axle is straightforward underthe assumption that the friction at each piston is the same. Whenoperating in “one valve-in-hold” mode there will be a pressure dropacross the balance valve during rapid pressure rise or dump, but anydifference during slow rates of pressure change will be due to sensorerrors.

b. Diagnosis requires a comparison between more than two sensors, butthe seal friction of the isolating pistons complicates an inter-axleplausibility check. Seal friction can be as much as 6 bar which islarger than the errors which should be detected. However when thepressure is changing it becomes possible to diagnose small errors if thefriction effect is taken into account.

c. Depending upon the sense of the error (+ve or −ve) it may not berecognized until the demand pressure changes by an amount greater thanthe seal friction. This technique can be used with any of 1. or 2.described above. FIG. 2 is a sequence flow chart illustrating theaforegoing two options 1 and 2.

FIG. 2 includes the following sequence steps:

40—start

41—Is V_(Ref)=0?

42—Is a sensor fault awaiting diagnosis?

43—Is start-up check in progress?

44—Was V_(Ref)>0 in preceding cycle?

45—Close accumulator isolating valve

46—Open all proportional control valves

47—Open all proportional control valves

48—Open accumulator isolating valve

49—Read signal from 4×brake and 1×supply pressure sensors

50—Are all signals same +/−×bar?

51—Note sensor identity, error magnitude and sense

52—Restore previous valve control status

53—Finish

An example of a brake pressure sensor plausibility check to establishwhether a faulty brake pressure sensor exists (but which is unable toidentify which particular sensor is faulty) is illustrated in FIG. 3.FIG. 3 includes the following sequence steps:

54—Start

55—Read all 4 brake pressure sensors

56—Balance valves open?

57—Cross-axle threshold exceeded at either axle?

58—Set “detection” flag for faulty axle

59—Set “waiting for diagnosis” flag

60—Safe to begin diagnosis?

61—Begin diagnosis test

62—Calculate inter axle threshold=seal friction+cross-axle threshold

63—Compare sensor readings between sensors on each diagonal or alongeach side of the vehicle

64—Inter-axle threshold exceeded at either side?

65—Set “diagnosis” flag for faulty axle at faulty side

66—Start

What is claimed is:
 1. A method of detection and identification ofpressure sensor faults in a braking system of the type comprising abrake pedal, respective braking devices connected to the vehicle wheelsand which communicate with electronically controlled valves in order toapply hydraulic fluid under pressure to the braking devices, respectivepressure sensors for measuring the hydraulic pressures at the individualbraking devices, a hydraulic pump driven by an electric motor, a highpressure hydraulic pressure accumulator fed by said pump for theprovision of hydraulic fluid under pressure which can be passed to thebraking devices via the electronically controlled valves in order toapply hydraulic fluid under pressure to the braking devices as afunction of the driver's braking demand as sensed at the brake pedal,and a supply pressure sensor for monitoring the hydraulic pressuresupplied to the electronically controlled proportional control valves,in which method at least three of said pressure sensors are subjected tothe same pressure and their readings are measured and compared toidentify a sensor whose reading does not correspond to that of theothers.
 2. A method according to claim 1, wherein in the case of avehicle having at least four or more wheels, all pressure sensorsassociated with the braked wheels, and the supply pressure sensor, areconnected together, with all of the proportional control valves eitherone of opened fully and substantially fully.
 3. A method according toclaim 1, wherein with the vehicle stationary and an accumulatorisolating valve open, a comparison of the readings from all sensors ismade under pressure.
 4. A method according to claim 2, wherein with thevehicle stationary and an accumulator isolating valve open, a comparisonof the readings from all sensors is made under pressure.
 5. A methodaccording to claim 1, wherein the comparison of the sensor readings ismade with the vehicle stationary and an accumulator isolating valveclosed.
 6. A method according to claim 2, wherein the comparison of thereadings from all sensors is made under pressure.
 7. A method accordingto claim 1, wherein a comparison is made of the brake pressures asmeasured by the pressure sensors, both across the axles and betweenfront and rear axles.
 8. A braking system comprising a brake pedal,respective braking devices connected to the vehicle wheels and whichcommunicate with electronically controlled valves in order to applyhydraulic fluid under pressure to the braking devices, respectivepressure sensors for measuring the hydraulic pressures at the individualbraking devices, a hydraulic pump driven by an electric motor, a highpressure hydraulic pressure accumulator fed by said pump for theprovision of hydraulic fluid under pressure which can be passed to thebraking devices via the electronically controlled valves in order toapply hydraulic fluid under pressure to the braking devices as afunction of the driver's braking demand as sensed at the brake pedal, asupply pressure sensor for monitoring the hydraulic pressure supplied tothe electronically controlled valves, and sensor control and comparisonmeans for subjecting at least three of said pressure sensors to the samepressure and measuring and comparing their readings so as to identify asensor whose reading does not correspond to that of the others.
 9. Asystem according to claim 8, wherein in the case of a vehicle having atleast four wheels, the control means can connect together all pressuresensors associated with the brake wheels, and the supply pressuresensor, with all of the proportional control valves either one of openedfully and opened substantially fully.
 10. A system according to claim 8,wherein, with the vehicle stationary and an accumulator isolating valveopen, the control means is arranged to effect a comparison of thereadings from all sensors under pressure.
 11. A system according toclaim 9, wherein with the vehicle stationary and an accumulatorisolating valve open, the control means is arranged to effect acomparison of the readings from all sensors under pressure.
 12. A systemaccording to claim 8, wherein said control means is arranged to effectthe comparison of the sensor readings with the vehicle stationary and anaccumulator isolating valve closed.
 13. A system according to claim 9,wherein said control means is arranged to effect the comparison of thesensor readings with the vehicle stationary and an accumulator isolatingvalve closed.
 14. A system according to claim 8, wherein the controlmeans is arranged to effect a comparison of the brake pressures asmeasured by the pressure sensors, both across the axles and betweenfront and rear axles.
 15. A system according to claim 8, wherein saidcontrol means comprises a system ECU.
 16. A method of detection andidentification of pressure sensor faults in an electro-hydraulic brakingsystem of the type comprising a brake pedal, respective braking devicesconnected to the vehicle wheels and which communicate withelectronically controlled proportional control valves in order to applyhydraulic fluid under pressure to the braking devices, respectivepressure sensors for measuring the hydraulic pressures at the individualbraking devices, a hydraulic pump driven by an electric motor, a highpressure hydraulic pressure accumulator fed by said pump via anisolating valve for the provision of hydraulic fluid under pressurewhich can be passed to the braking devices via the proportional controlvalves in order to apply hydraulic fluid under pressure to the brakingdevices in proportion to the driver's braking demand as sensed at thebrake pedal, and a supply pressure sensor for monitoring the hydraulicpressure supplied to the electronically controlled proportional controlvalves, in which method, with the accumulator isolating valve closed andwith the vehicle having been brought to a stationary condition byoperating the brake pedal and before the brake pedal is released, atleast three of said pressure sensors are arranged to be subjectedcommonly to the pressure then existing within the braking systemdownstream of the accumulator isolating valve and their readingsmeasured and compared whereby to identify a sensor whose reading doesnot correspond to that of the others.
 17. A method according to claim 1,wherein in the case of a vehicle having at least four or more wheels,all pressure sensors associated with the braked wheels, and the supplypressure sensor, are connected together, with all of the proportionalcontrol valves either one of opened fully and substantially fully.
 18. Amethod according to claim 1, wherein a comparison is made of the brakepressures as measured by the pressure sensors, both across the axles andbetween front and rear axles.
 19. An electro-hydraulic braking systemcomprising a brake pedal, respective braking devices connected to thevehicle wheels and which communicate with electronically controlledproportional control valves in order to apply hydraulic fluid underpressure to the braking devices, respective pressure sensors formeasuring the hydraulic pressures at the individual braking devices, ahydraulic pump driven by an electric motor, a high pressure hydraulicpressure accumulator fed by said pump for the provision of hydraulicfluid under pressure which can be passed to the braking devices via theproportional control valves in order to apply hydraulic fluid underpressure to the braking devices in proportion to the driver's brakingdemand as sensed at the brake pedal, a supply pressure sensor formonitoring the hydraulic pressure supplied to the electronicallycontrolled proportional control valves, and control means foridentifying a sensor whose reading does not correspond to that of theothers, which control means closes the accumulator isolating valve and,when the vehicle has been brought to a stationary condition by operatingthe brake pedal and before the brake pedal has been released, isarranged to measure the readings from at least three of said pressuresensors subjected to the common pressure then existing within thebraking system downstream of the accumulator isolating valve.
 20. Asystem according to claim 19, wherein in the case of a vehicle having atleast four or more wheels, the control means can connect together allpressure sensors associated with the brake wheels, and the supplypressure sensor, with all of the proportional control valves either oneof opened fully and opened substantially fully.